bus: simplify bus_message_read_strv_extend()

bus: simplify bus_message_read_strv_extend()

Avoid unbound for(;;) loop and use the established coding-style:
  while ((r = sd_bus_message_read*(...)) > 0) {
  }
  if (r < 0)
          return r;

This is much easier to read and used all over the code base.